In recent years, with the development of various kinds of compact and cordless electronic devices, it has been demanded for secondary batteries as power sources for driving them to have high capacity and decrease weight. As representative secondary batteries, there are well-known lead-acid batteries, alkaline storage batteries, lithium secondary batteries, etc. Since the lithium secondary batteries which are nonaqueous electrolyte secondary batteries utilizing a lithium ion doping and dedoping action can realize the high capacity especially among the above-described secondary batteries, they have been examined in various aspects.
For instance, for a lithium-ion secondary battery of the nonaqueous electrolyte secondary battery which can realize the demands of the secondary battery capable of having high capacity, low weight and high energy density and excellent in its charging and discharging cyclic characteristics, it has been desired to realize a practical secondary battery with a battery structure in which a battery performance is scarcely deteriorated even when the battery is used for a long time, and which employs stable electrodes and an electrode composite mixture and an electrode active material and a positive active material composite mixture hardly deteriorated even upon use under a condition of high temperature or for a change in the battery upon abnormality of the battery.
In case the above-described nonaqueous electrolyte battery has a sealed type structure, when the electric current of a prescribed quantity of electricity or more is supplied upon charging due to any cause so that the nonaqueous electrolyte battery is overcharged, battery voltage will rise and electrolyte solution or the like will be decomposed to generate gas so that the internal pressure of the battery will rise. When this overcharged state is continued, an abnormal reaction that the electrolyte or the active materials are rapidly decomposed is generated and the temperature of the battery abruptly rises.
As a measure for suppressing such a rise of the temperature of the battery, there is proposed an explosion-proof type sealed battery having a current cut-off means which operates in accordance with the rise of the internal pressure of the battery. In such an explosion-proof closed type battery, for instance, when an overcharged state advances to generate gas due to the chemical change of the inner part of the battery so that the internal pressure of the battery rises to a prescribed threshold value or higher, the current cut-off means operates in accordance with the rise of the internal pressure to cut off charging current so as to suppress the rapid rise of the temperature of the battery.
As described above, in order to operate the current cut-off means, the internal pressure of the threshold value or higher is required. However, in the nonaqueous electrolyte secondary battery, the decomposition of the electrolyte or the active materials may advance to generate heat which leads to the quick rise of temperature so that the current cut-off means may not effectively operate, before the internal pressure of the battery rises to reach the threshold value.
Thus, in order to assuredly operate the current cut-off means, there is put into practical use a method for including lithium carbonate of 0.5 wt % to 15 wt % in lithium composite oxide such as LiCoO2 serving as a positive active material as disclosed in Japanese Patent Application Laid-Open No. hei. 4-328278. According to this method, carbon dioxide gas generated when lithium carbonate is electrochemically decomposed serves to suppress an abnormal reaction during an overcharging operation. Further, since the battery is filled with not only gas generated as a result of the decomposition of electrolyte solution, but also carbon dioxide gas generated from lithium carbonate, the current cut-off means can be assuredly operated in an early stage and the rise of temperature of the battery can be advantageously assuredly suppressed.
However, when lithium carbonate is included in a cathode in order to obtain an assured suppressing effect for the rise of temperature of the battery, as described above, a battery capacity has been inconveniently deteriorated.
Further, for example, in a conventional nonaqueous electrolyte battery as disclosed in Japanese Patent Application Laid-Open No. hei. 8-45498, there has been a problem that charging and discharging cyclic characteristics cannot be sufficiently improved depending on the particle diameter of the lithium manganese oxide and lithium nickel oxide. Further, in the nonaqueous electrolyte battery, since battery characteristics such as an initial capacity are deteriorated depending on the selection of an negative active material, especially, a larger deterioration is generated upon storage of the battery, there is left room for improvement of the battery characteristics.
Still further, in the nonaqueous electrolyte battery, especially when LiNiO2 is employed as the positive active material, an expansion and contraction are generated upon charging and discharging operations like graphite or other alloys used as the negative active material. Thus, the volume change of the positive active material is generated, and accordingly, there inconveniently arises a phenomenon that a composite mixture layer including the active materials is peeled off or electrodes are deformed as charging and discharging cycles advance to deteriorate the cyclic characteristics.